Conventionally, measurement of hemoglobin concentration in blood is performed by hemolyzing sampled blood physically or chemically, introducing the sample to a cuvette and irradiating it with light of a specific wavelength, measuring the transmitted light and calculating the hemoglobin concentration using the Lambert-Beer law. With an apparatus and method for measuring the degree of oxygen saturation in hemoglobin contained in blood, the blood is irradiated with light having two wavelengths .lambda..sub.1, .lambda..sub.2, the intensity of the reflected light is measured, and the degree of oxygen saturation is determined from the following equation: EQU SO.sub.2 =A+B.times.(I.sub.2 /I.sub.1)
where I.sub.1, I.sub.2 represent the intensities of the reflected light at the respective wavelengths .lambda..sub.1, .lambda..sub.2, and A, B are constants.
A problem with the above-described method of measuring hemoglobin concentration in blood is that continuous measurement is difficult since it is necessary to hemolyze the blood measured. In addition, a problem with the above-described method of measuring the degree of oxygen saturation of hemoglobin is that the results of measuring oxygen saturation are prone to error owing to the significant influence of physiological factors in blood, especially hematocrit value (the proportion of blood occupied by red blood cells). More specifically, the extinction (reflection) characteristics of blood vary depending upon absorption and scattering caused by pigments and particles contained in the blood. In particular, as shown in FIG. 13, light-absorption coefficient varies greatly depending upon the state of bonding between hemoglobin and oxygen and the wavelength of the irradiating light. Here HbO.sub.2 represents oxygenated hemoglobin, Hbr represents reduced hemoglobin, and HbCO stands for carbomonoxyhemoglobin.
In the vicinity of a wavelength of 800 nm, Hb0.sub.2 and Hbr intersect and the light absorbancies are equal, as will be understood from these drawings. This wavelength is referred to as a point of equal absorption. This indicates a wavelength at which light absorbancy is not changed by the degree of oxygen saturation of hemoglobin.
FIGS. 14A and 14B are graphs in which the relationship between reflected-light intensity at wavelengths of 660 nm and 800 nm, respectively, and the degree of oxygen saturation is plotted while varying the hematocrit value (HCT) and hemoglobin (Hb) concentration. The blood sample used here was bovine blood.
In the case of the wavelength of 660 nm shown in FIG. 14A, the light absorbancy of oxygenated hemoglobin is small in comparison with that of reduced hemoglobin. Consequently, reflected-light intensity increases with a rise in the degree of oxygen saturation. In the case of the wavelength of 800 nm shown in FIG. 14B, it will be understood that a change in degree of oxygen saturation does not have much influence because this wavelength is the equal absorption point. Furthermore, it will be understood from FIGS. 14A, 14B that reflected-light intensity decreases with a decrease in the hematocrit value at each wavelength.
It should be noted that these measurements of reflected-light intensity are results obtained upon previously calibrating each reflected-light intensity to a predetermined value using a white reflector.
FIG. 15 illustrates the relationship between degree of oxygen saturation calculated using the foregoing equation and degree of oxygen saturation measured using an OSM2 hemoxymeter (manufactured by Radiometer) from measured values of reflected-light intensity at wavelengths 660 nm and 800 nm shown in FIGS. 14A, 14B, respectively. As a result, the degree of oxygen saturation calculated from the foregoing equation is profoundly influenced by the hematrocrit value in the region of low oxygen saturation, and a large error develops in the calculated value of oxygen saturation. Furthermore, in the prior art, the degree of oxygen saturation in blood cannot be measured accurately in continuous fashion without being influenced by the hematrocrit value.